A new drug has shown some promise in patients with mild dementia, and might be even more effective if given to those at risk of the nuerodegenerative disease long before symptoms appear, researchers said.

Scientists are "full of hope" that a breakthrough in drug therapy to prevent dementia could come within five years, said Dr Eric Karran, director of research at Alzheimer's Research UK.

Karran said if further trials on the drug succeed, it could mean that those with a family history of dementia are given monthly injections of the drug a decade before any signs of Alzheimer's appear.

Trials suggest that the drug called solamezumab may delay the onset of disease, halting problems with brain function and behaviour in those with mild dementia.

Researchers originally tested the drug on patients with mild to moderate dementia, where the treatment did not achieve effective results. However, when analysis examined the impact of the drug only on those with mild dementia, researchers found it had an effect both on their daily behaviour and the functioning of their brain and memory. People aged 55 to 90 are currently being recruited to a final-stage study in the US, which will examine if the drug slows the cognitive and functional decline of people with mild Alzheimer's.

Given that changes in the brain associated with dementia occur as far as a decade before symptoms appear, Kerran said if the drug trials succeed, it would be "logical" to prescribe them to patients. "I am full of hope that we are going to have a breakthrough in five years," said Kerran.

The only drugs presently used for dementia can mask the symptoms, but do not delay the onset of disease.